Suppose there are several other children in the family.
Her nine year old brother. He has some understanding of ideas like "being marriageable," (although he may not fully understand their importance), and has begun reading some things like the Analects (er... I'm not entirely sure whether a nine year old boy in a well-bred Han family would be reading the Analects. What would be appropriate at that age?). He could probably give a short, shallow answer about why foot-binding is desirable -- that it makes his sister more ladylike, refined, marriageable, etc, but when he sees the agonizing and disabling process carried out on his sister, he wants to ask his mother (or father) a lot of questions that start with "why?"
Her four year old sister. She's heard this is something that will happen to her too. Seeing how awful it seems, she starts thinking she wants to avoid it, and finds the prospect very frightening.
Her four year old brother (twin of the sister). He also is confused (and maybe a little frightened) by what's happening.
How would the parents explain this custom to the other children in the family?
The answer for all of them would be roughly the same, but would depend somewhat on the family and their social standing. The answer might also differ from mother to father, but would most likely follow this general answer: Having small feet will give you more opportunities in life. One of the first things a man will look for in a good bride will be her feet. Binding one's feet is not a punishment but makes the girl more beautiful and desirable in marriage. If a women does not bind their feet they will have no opportunities to marry a respectable man.
A child's goal in life is to be filial and serve their parents. A mother bore many burdens and the family must feed and clothe you, so you owe your parents a great debt that can never be repaid. The Analects reinforce this view. Look at the Analects book Two which discuss what filial means. Specifically Book 2.5 "Never Disobey" (in regards to parents)